A helix resonator or a helix is a transmission line resonator with a physical length of about one quarter of a wave length. It is well known to use a helix resonator as a tuning element and it is widely used in filters in the high frequency band, particularly at 100 to 1000 MHz. A resonator of this kind comprises inductive elements; a conductor wound to a helical coil and a metallic housing at a distance from the coil. The low impedance (grounded) end of the coil is usually connected directly to the metallic housing. The opposite end, the high impedance end of the coil is separated from the housing and capacitively coupled to it. A connection to the resonator can be made in a known way by soldering a signal conductor directly to the helix coil, usually to the first turn of the coil. This connecting point determines the impedance level of the resonator, and thus the resonator can be matched to the rest of the circuit by the selection of this point. This matching, in which the connection point forms a tap of the resonator coil, is called tapping and this point is called the tapping point. The tapping point can be calculated or determined experimentally.
The characteristic impedance of a helix resonator is determined by the ratio of the diameter of the coil to the inside dimension of the housing surrounding the coil, by the mutual distance of the turns in the coil or the so called pitch, and by any dielectric material used as a support for the resonator. The resonance frequency of a helix resonator is a function of the physical dimension of the coil, of the capacitive structure, and of the distance between the high impedance end and the housing. Thus an accurate and exact design is required to manufacture a resonator with a specified frequency band.
A filter can be manufactured by placing several helix resonators in the same housing. A compartment housing can be used to control the electromagnetic coupling between adjacent resonators, in other words, each resonator is placed in its own compartment so that there is a partition of the housing between adjacent resonators. When an aperture, a so called coupling aperture is made in the partition so that the aperture has a defined size and a defined position, we obtain the desired coupling factor k between the resonators which indicates how much electromagnetic energy passes through the partition aperture from one resonator to the other.
FIG. 1 is a simplified representation of a filter housing having a coupling aperture in a partition. FIG. 1 shows in a simplified way the housing 1 of a filter realized with helix resonators, the housing comprising a cover 2, side surfaces 3 and end surfaces. The bottom of the housing is open and the cylindrical coils are mounted in the housing through its bottom. In this example the housing has three partitions 4, 5, 6, which divide the internal space into four compartments c1, c2, c3 and c4. At least one partition, the partition 4 in the figure, has a coupling aperture 7.
FIG. 2 shows how the coupling aperture is made in accordance with the prior art. In the current production filters the coupling apertures are located in the centre of the partition and according to FIG. 2 made with aperture tools comprising a die and a cushion. According to the figure showing a filter as seen from one side, the aperture tool is put inside the filter housing via the open bottom of the multi-compartment filter housing 21 so that the partition 22 remains between the die 23 and the cushion 24. Then the die is forced against the cushion, whereby an aperture is cut in the partition, the aperture being e.g. in accordance with the aperture 7 of FIG. 1.
An advantage of this known way to make an aperture is that the use of the same tool always produces an aperture of the same size and in the same place, which means good reproducibility. Further in a cutting line, which is a series of consecutive operations by which the filter housing is mechanically processed, the aperture making step is rather short.
The disadvantage of this known way to make an aperture is above all that a new die and cushion have to be made for different aperture sizes. The tool is manufactured by wire quenching of an annealed billet, thus it is a slow and expensive operation to make a new die and cushion. Secondly, as the filter size decreases, e.g. to filters lower than 4 mm, it is questionable whether it is possible to make apertures with this aperture tool method at all. This is because the durability of the tool becomes a problem as, in order to make a functional tool, the dies and cushions must be so thin and narrow that they can not withstand the stresses caused by cutting in mass production and fail. The fracture positions are shown in FIG. 2 by wave lines a and b.